Much technology has already been developed concerning composite gas separation membranes. Fundamentally, the purpose for a composite (or "multiple-layer") membrane structure is to allow the selection and combination of multiple materials which can each perform some of the necessary functions of the overall membrane better than any one of the materials could perform all of such functions. The process of selective passage of certain types of molecules in gaseous phase through a nonporous membrane material is a complex phenomenon occurring on a molecular level. Generally, the molecular selectivity is a combination of diffusion through the membrane material (controlled by the packing and molecular free volume of the material), and gas solubility within the membrane material. Returning to the diffusion concept, the selective membrane material must have the very special performance property that certain types of molecules preferentially will pass through it, resulting in a concentration of such types of molecules on the permeate side of the membrane. Such selective membrane materials can be very expensive to develop and produce, and accordingly they command a high price. Further, since the gas molecules must physically pass through the selective membrane itself, overall membrane flux will be maximized when the selective membrane thickness is minimized. This is a crucial consideration in designing a gas separation membrane, because higher flux translates into higher productivity. Lower flux directly results in greater compression requirements to force the gas through the membrane, translating into increased operating costs. As a result of these considerations, the gas molecule selection function of a gas separation membrane is best performed by an ultrathin layer of a specially-selected, often expensive selective membrane material.
Another important function of a gas separation membrane is to withstand the pressure drop across the membrane which is encountered in and necessary for its operation, and otherwise endure a reasonable lifetime as an integral material in the intended operating environment. This function is best performed by a structural support material which (1) can be prepared economically as a relatively thick layer which will provide adequate mechanical strength, and (2) is highly permeable, so as not to markedly reduce the gas flux of the overall membrane.
Ideally, the selective membrane material is directly placed over the structural support material. The two-layer composite then combines optimum selectivity, gas flux and mechanical durability. However, disclosures have been made of many circumstances in which a gutter layer is interposed between the selective membrane material and the structural support material, in order to facilitate and improve the combination of such structural support and selective membrane materials. For example, the Browall et al U.S. Pat. No. 3,874,986 discloses the interposition of an organopolysiloxane-polycarbonate copolymer layer as a gutter between a microporous backing material (such as Acropore polyvinyl chloride-acrylonitrile copolymer) and an ultrathin polyphenylene oxide (PPO) selective membrane; the gutter overcomes delamination problems by adhering both to the PPO layer and to the Acropore backing. (We hereby incorporate by reference the complete contents of every patent and every other document which is mentioned anywhere in this application). See similarly, the Browall U.S. Pat. No. 3,980,456. The Cabasso et al U.S. Pat. No. 4,602,922 discloses a composite gas separation membrane made by in situ crosslinking of an aminoorganofunctional polysiloxane with a diisocyanate to constitute a gutter layer on the surface of a highly porous polymer substrate such as polysulfone or polystyrene; a gas separating entity such as polyphenylene oxide can then be coated on the gutter layer. (Cabasso discusses the two abovementioned Browall patents at column 7, lines 33-39).
Despite these advances, the need to further develop the composite gas separation membrane art continues, driven in part by incompatibility problems between particular polymeric materials which desirably could be incorporated in composite structures. One class of such polymers are those referred to as the "6FDA polyimides". These polymers can be formed (as illustrated in FIG. 1 of this application) by (A) the condensation of 5,5'-2,2,2-trifluoro-1-(trifluoromethyl)ethylidene-bis-1,3-isobenzofuraned ione (Formula I, known as "6FDA") with an aromatic diamine such as 1,3-diaminobenzene (Formula II) or 1,5-naphthalenediamine (Formula III); and (B) dehydration to yield a 6FDA polyimide (Formula IV). The value and applicability of 6FDA polyimides as gas separation membranes is well known and documented, e.g., in the Hoehn et al U.S. Pat. Reissue No. 30,351 (based on U.S. Pat. No. 3,899,309), the Hayes U.S. Pat. No. 4,717,394, and the Ekiner et al U.S. Pat. No. 5,085,676. Asymmetric monolayer films of these polymers, moreover, can be made. However, the expensive status of these polymers virtually mandates a composite membrane structure.
The Hayes U.S. Pat. No. 4,717,394 discloses certain classes of 6FDA-type-polyimide polymers (we mean to indicate by the term "6FDA-type polyimide" that 6FDA can be replaced by another dianhydride having similar molecular structure and activity; and that the aromatic diamine can also be varied), and gas separation membranes fabricated from them. Hayes discusses the impact of relative polymer chain rigidity on membrane gas permeance, and presents means for the controlled addition of reduced chain rigidity, asserting that these modifications allow for improved membrane selectivity while still maintaining high permeance to gases. Hayes discloses that the polymers can be solution cast on a porous solvent-resistant substrate to serve as the dense separating layer of a composite membrane. In Examples 1-21, the polymers are cast on a glass plate to form a cast membrane film; in Examples 22 -36, the polymers are again cast on glass, but asymmetric membranes are formed.
The Ekiner et al U.S. Pat. No. 5,085,676 is the most recent patent of which applicant is presently aware that relates to the preparation of 6FDA-type-polyimide gas separation membranes. According to Ekiner, two or more film-forming polymer solutions are simultaneously coextruded to form a nascent membrane, followed by precipitation to form a composite multicomponent membrane comprised of a dense or asymmetric gas separating layer and a microporous layer which structurally supports the separating layer. The membrane is then quenched and the remainder of the solvent is removed to form a gas separation membrane. The substrates for the 6FDA-type-polyimide polymers in Ekiner's Examples include: glass (from which a solidified membrane film is then stripped); polyether sulfone/polyvinylpyrrolidone flat sheet substrates; polyether sulfone/polyvinylpyrrolidone hollow fiber substrates; and polyamide hollow fiber substrates.
The Hoehn et al U.S. Pat. Reissue No. 30,351 (based on U.S. Pat. No. 3,899,309), discloses certain classes of aromatic polyimides, polyesters and polyamides and their fabrication into gas separation membranes. Hoehn discloses, at column 4, lines 39-54, that these polymers can be solution cast on a support to produce a sheet membrane or spun through a spinneret to yield hollow fibers. Hoehn notes that both uniform and asymmetric membranes can be made. In Hoehn's Examples concerning polyimide membrane preparation, the polymer is cast onto glass or Inconel and then stripped from it.
In spite of these developments, the need continues for improvements in membranes having ultrathin 6FDA-type polyimide selective membrane layers. In particular, composite structures which will take advantage of the demonstrated gas separation utility of 6FDA-type-polyimide polymers, while avoiding the need for great amounts of the expensive 6FDA-type-polyimide polymers, would be desirable.